I purposely repeated the OP's wording but since we're talking about an astable multivibrator,frequency and duty cycle (in percent) makes sense. I did a lot of circuit design for communications systems and I associate the words mark and space with ones and zeroes in a frame/word.

I wondered if it was an English (not American) phrasing.

More on the origins of the terminology here: https://en.wikipedia.org/wiki/Mark_and_space